The present invention relates to an optical information reading apparatus for reading recorded information from a disk-like recording medium such as a video disk, a digital audio disk, or the like (hereinafter simply referred to as a "disk").
In such an apparatus, a focusing servo for bringing a light beam radiated from an .optical pickup into accurate focus, and a tracking servo for making a beam spot accurately trace a recording track are indispensable for reading recorded information accurately from a disk. In these types of apparatus, if a disk is warped, the information recording surface of the disk becomes tilted relative to an optical axis of an information detecting light beam, and a tracking error signal is generated, since the shape of the beam spot is warped by factors such as a comatic aberration, even when the beam spot is correctly on a recording track. Under these circumstances, the tracking servo can shift the beam spot out of the recording track, and will provide an indication of zero tracking error, despite this mispositioning. The beam spot therefore is not radiated correctly on the recording track in spite of the tracking error being "officially" zero, so that operation may be influenced by pit information from an adjacent track, resulting in crosstalk.
In order to prevent such deterioration in performance, a tilting servo action may be performed, in which tilt of an optical axis of an information detecting light beam relative to an information recording surface of a disk is detected and pickup tilt is adjusted so that the optical axis of the light beam is kept always perpendicular to the information recording surface.
FIG. 5 shows a conventional tilting servo device in which an optical pickup 3 for reading recorded information of a disk 2 which is driven and rotated by a spindle motor 1 includes an objective lens 4. The objective lens 4 is mounted on an optical body 5 for movement in the direction of an optical axis A--A', the objective lens 4 being driven by a focus actuator (not shown). The optical body 5 is attached to a slider base 6 for rotation about a pivot 0 positioned on the optical axis A--A' of the objective lens 4. An adjustment mechanism 7 constituted by a tilt motor M (shown in FIG. 6), a speed reduction mechanism, etc., rotates the optical body 5 relative to the slider base 6 to adjust tilt of the optical axis A--A' of the objective lens 4 relative to an information recording surface. The slider base 6 is movable in a radial direction B--B' of the disk 2 along a guide rail (not shown), and is driven by a driving mechanism (not shown), for example, a slider motor, a reduction gear, etc.
In a plane including the optical axis A--A' of the objective lens 4 and perpendicular to the moving direction of the slider base 6, a tilt sensor 8 for detecting a tilt angle of the disk 2 relative to the optical axis A--A' is provided in the vicinity of the objective lens 4. As shown in FIG. 6, the tilt sensor 8 is constituted by a luminous element 9 and two light receiving elements 10a and 10b, so that the light receiving elements 10a and 10b can receive a light beam which is emitted from the luminous element 9 and then is reflected from the information recording surface of the disk 2. A difference between respective intensities of light received by the light receiving elements 10a and 10b is obtained by a differential amplifier 11, and is supplied, as information related to tilt of the disk 2 relative to the optical axis A--A', to a tilt motor 13 in the adjustment mechanism 7 (shown in FIG. 5) through a driving amplifier 12.
In the FIG. 5 arrangement, when the slider base 6 moves from a position (a) to a position (b) corresponding to a warped portion on the outer circumference of the disk 2, a tilting servo action is performed such that the adjustment mechanism 7 is driven in response to a driving signal in accordance with an output of the tilt sensor 8 so that the optical body 5 is rotated clockwise in FIG. 5 about the pivot 0 to make the optical axis A--A' perpendicular to the information recording surface of the disk 2.
In such a conventional apparatus, tilt of the optical axis of the information detecting light beam relative to the information recording surface of the disk 2 is detected by the tilt sensor 8 provided in the vicinity of the objective lens 4, and the tilt is corrected so that the beam spot is positioned accurately on the recording track. A detection error therefore is large because of a difference between the irradiation positions of the information detecting light beam and the light beam for the tilting servo.
If there is an attachment error or the like in the tilt sensor 8, it is impossible to detect tilt of the optical axis of the information detecting light beam with respect to the recording surface, even if it is possible to detect tilt of the optical axis of the light beam for the tilting servo relative to the information recording surface of the disk 2. Consequently, it has been impossible to correct accurately a displacement of the beam spot relative to the recording track caused by the tilt of the optical axis of the information detecting light beam. Further, because of a requirement of providing an optical system (the tilt sensor 8) for detecting tilt independently of an optical system for detecting information, not only is there an increase in the number of parts, but it is also necessary to provide sufficient space for the additional parts, so that the apparatus is large in size and is expensive.